The invention relates to an automatic sewing machine having a transport apparatus for transporting a band element to the foot of a sewing machine.
Such an automatic sewing machine is used, for example, to sew rubber bands onto leg sections and onto the waistband of tricot fabrics. There are also known automatic sewing machines in which the fabric itself is transported with the aid of a band driven synchronously with the sewing drive. The invention is also used for this type of "bands".
Here, the term "automatic sewing machine" designates a system which essentially consists of a conventional sewing machine and additional means which are designed for special applications of the automatic sewing machine. Such additional means may be, for example, a special fabric feed means for feeding the fabric to the sewing point, and other possible additional means are a cutting apparatus (thread and band cutter) or an apparatus for inserting and sewing on labels.
The motor and its control circuit may be of the conventional type, for example a controllable direct current motor. A particularly interesting additional means here is a means which is generally referred to by the term "metering device" and which is specially designed in the form of so-called metering rollers, for synchronously feeding an element to be transported, namely a band.
In an automatic sewing machine of the type stated at the outset, such a metering device ensures metering of bands, for example rubber bands. These bands are transported to the sewing point under the pressure foot, so that they come to rest at the edge of the fabric and are sewn to the fabric edge. In the simplest case, fabric transport and the feed rate of the band are identical, so that there is no tension at all between fabric and band.
However, especially when sewing on rubber bands, frequently a difference is expressly required between the rate of advance of the fabric on the one hand and the feed rate of the rubber band on the other hand, in order to achieve a certain gathering or crimping of the fabric in the region of the rubber band. Previously, automatic sewing machines had a mechanical coupling between the sewing machine drive and the metering device. In modern automatic sewing machines, such a mechanical coupling is replaced by an electronic coupling: in general, a pulse generator on the sewing machine opto-electrically scans the speed of the sewing machine drive and sends a speed-dependent pulse signal to a signal processing means, which in turn controls the driver circuit of the stepping motor of the metering device. In principle, however, other (for example analogue) position or speed signals are also possible, as are different motors, which are synchronous or direct current motors.
The electronic coupling between the sewing machine and the band metering device has a number of advantages. The gathering or crimping of the fabric which can be achieved by different speeds of the fabric on the one hand and the band on the other hand depends on the position at which the band is sewn onto the fabric.
For different fabric regions, it is frequently necessary to provide different rates of advance of the band in order to achieve different tensions at the relevant sewing points. In the case of the electronic coupling between the sewing machine and the metering device, the desired band tension relative to the fabric can be easily preset with the aid of a simple switch arrangement, with matrix plugs, potentiometers or key pads.
The ratio of fabric advance to tape metering is constant if it is assumed that the stitch length of the sewing machine is independent of the speed of the sewing machine drive.
However, it has now been found that the stitch length L is not independent of the speed of the sewing machine drive but increases with increasing speed. FIG. 1 shows the dependence of the stitch length L, measured in millimeters, on the speed of the sewing machine drive, measured in rpm, by way of example in a graphical representation. In the ideal case, there would be a constant stitch length L of, for example, 2 mm, independent of the speed. The stitch length L multiplied by the number of stitches per unit time gives the advance of the fabric. With a constant stitch length L, a fabric advance proportional to the speed was thus also obtained. The pulse generator coupled to the sewing machine drive sends a correspondingly large number of pulses per unit time at high speeds. Accordingly, the metering device delivers larger amounts of band at higher speeds.
If the stitch length is considered in relation to the fabric on the one hand and in relation to the band on the other hand, the conditions shown in FIG. 1 are obtained:
In the ideal case, regardless of the speed, the stitch length L at the fabric on the sewing machine is constant and, for example, has a value of 2.0 mm. Hence, in the ideal case, the stitch length L has a constant value of, for example, 2.0 in relation to the rubber band, once again regardless of the speed.
In practice, however, the stitch length is not constant but changes with increasing speed of the sewing machine drive, the change generally being an increase. This is indicated in FIG. 1 by a -- ' -- line Nr, which deviates from the ideal line N.sub.i.
In the older sewing systems of the type under discussion here, having mechanical coupling between the sewing machine and the metering device (MD), there was, so to speak, an automatic correction by virtue of the fact that the metering device also delivered correspondingly more band at higher speeds, i.e. a disproportionately large amount of band relative to the speed. There were therefore virtually no defects in the sewn article since the desired tension between the fabric and the band was achieved in each case.
If the speed is now picked up at the sewing machine drive by means of a pulse generator and this speed is converted into control signals for an electric stepping motor, the metering device delivers twice as much band per unit time at twice the speed. Thus, the stitch length L remains constant in relation to the metered band while the stitch length L of the sewing machine (in relation to the transported fabric) changes at higher speed, whereby the resulting error curve can be very different, but will rise in many cases.
Due to the above-mentioned circumstances, the sewn product exhibits defects in so far as the desired tension, which is set differently depending on the sewn material, between the fabric and the band is irregular, i.e. too high at some points and too low at other points.
In FIG. 1, the metering with the aid of a mechanically coupled metering device is designated by MD mech. The dash-dot line parallel to the abscissa indicates the ideal relationship of sewing machine speed and stitch length L of the sewing machine, as well as the relationship between stitch length L, relative to the band, and the speed of the sewing machine.